In philosophy, ontology (from the Greek ὄν, genitive ὄντος: of being (part. of εἶναι: to be) and -λογία: science, study, theory) is the study of being or existence. It seeks to describe or posit the basic categories and relationships of being or existence to define entities and types of entities within its framework. Ontology can be said to study conceptions of reality.

Some philosophers, notably of the Platonic school, contend that all nouns refer to entities. Other philosophers contend that some nouns do not name entities but provide a kind of shorthand way of referring to a collection (of either objects or events). In this latter view, mind, instead of referring to an entity, refers to a collection of mental events experienced by a person; society refers to a collection of persons with some shared characteristics, and geometry refers to a collection of a specific kind of intellectual activity. Any ontology must give an account of which words refer to entities, which do not, why, and what categories result. When one applies this process to nouns such as electrons, energy, contract, happiness, time, truth, causality, and God, ontology becomes fundamental to many branches of philosophy.

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Ontology has one basic question: "What actually exists?" Different philosophers provide different answers to this question.

One common approach is to divide the extant entities into groups called "categories". However, these lists of categories are also quite different from one another. It is in this latter sense that ontology is applied to such fields as theology, information science and artificial intelligence.

The concept of ontology is generally thought to have originated in early Greece] and occupied Plato and Aristotle. While the etymology is Greek, the oldest extant record of the word itself is the Latin form ontologia, which appeared in 1606, in the work Ogdoas Scholastica by Jacob Lorhard (Lorhardus) and in 1613 in the Lexicon philosophicum by Rudolf Göckel (Goclenius).
The first occurrence in English of "ontology" as recorded by the OED appears in Bailey’s dictionary of 1721, which defines ontology as ‘an Account of being in the Abstract’. However its appearance in a dictionary indicates it was in use already at that time. It is likely the word was first used in its latin form by philosophers based on the latin roots, which themselves are based on the Greek.

Students of Aristotle first used the word 'metaphysica' (literally "after the physical") to
refer to the work their teacher described as "the science of being qua being". The word 'qua' means 'in the capacity of'. According to this theory, then, ontology is the science of being inasmuch as it is being, or the study of beings insofar as they exist. Take anything you can find in the world, and look at it, not as a puppy or a slice of pizza or a folding chair or a president, but just as something that is. More precisely, ontology concerns determining what categories of being are fundamental and asks whether, and in what sense, the items in those categories can be said to "be".

Ontological questions have also been raised and debated by thinkers in the ancient civilizations of India and China, in some cases perhaps predating the Greek thinkers who have become associated with the concept.

"What exists", "What is", "What am I", "What is describing this to me", all exemplify questions about being, and highlight the most basic problems in ontology: finding a subject, a relationship, and an object to talk about. During the Enlightenment the view of René Descartes that "cogito ergo sum" ("I think therefore I am") had generally prevailed, although Descartes himself did not believe the question worthy of any deep investigation. However, Descartes was very religious in his philosophy, and indeed argued that "cogito ergo sum" proved the existence of God. Later theorists would note the existence of the "Cartesian Other" — asking "who is reading that sentence about thinking and being?" — and generally concluded that it must be God.

This answer, however, became increasingly unsatisfactory in the 20th century as the philosophy of mathematics and the philosophy of science and even particle physics explored some of the most fundamental barriers to knowledge about being. Sociological theorists, most notably George Herbert Mead and Erving Goffman, saw the Cartesian Other as a "Generalized Other," the imaginary audience that individuals use when thinking about the self. The Cartesian Other was also used by Freud, who saw the superego as an abstract regulatory force.

Schools of subjectivism, objectivism and relativism existed at various times in the 20th century, and the postmodernists and body philosophers tried to reframe all these questions in terms of bodies taking some specific action in an environment. This relied to a great degree on insights derived from scientific research into animals taking instinctive action in natural and artificial settings — as studied by biology, ecology, and cognitive science.

The processes by which bodies related to environments became of great concern, and the idea of being itself became difficult to really define. What did people mean when they said "A is B", "A must be B", "A was B"...? Some linguists advocated dropping the verb "to be" from the English language, leaving "Prime", supposedly less prone to bad abstractions. Others, mostly philosophers, tried to dig into the word and its usage. Heidegger attempted to distinguish being and existence.

Existentialism regards being as a fundamental central concept. It is anything that can be said to 'be' in various senses of the word 'be'. The verb to be has many different meanings and can therefore be rather ambiguous. Because "to be" has so many different meanings, there are, accordingly, many different ways of being.

The first formal development of this notion within philosophy began with the pre-SocraticHeraclitus, where he posited agon ("strife of opposites") as the ontological basis of all reality in terms of this endless transformative conflict, which was later contrasted and dominated by the Parmenidean, or Platonic, notion of Being, until more recent philosophers began a reversion of this trend.

Notably and the first to make such an advocation since Heraclitus was the nineteenth century German philosopher Friedrich Nietzsche, who used the expression "the innocence of becoming", a fundamental element of his philosophical thought grounded in the "will to power as pathos", as a means to describe the aesthetic qualities of existence, which pervades his thinking, including but not limited to ideas such as his "Dionysian world", "eternal recurrence", "amor fati", and "decadence". It was with this a-teleological view that he attempted to disgregate all views pertaining to the human condition, where "thingness" is ultimately characterized as a mere "hypothesis" in Nietzsche's phrase, and such a view, pertaining to the "inequality" of all "things", carries deep implications for ethics and the nature of knowledge.

Social scientists adopt one of four main ontological approaches: realism (the idea that facts are out there just waiting to be discovered), empiricism (the idea that we can observe the world and evaluate those observations in relation to facts), positivism (which focuses on the observations themselves, attentive more to claims about facts than to facts themselves), and postmodernism (which holds that facts are fluid and elusive, so that we should focus only on our observational claims).